Since the start of digital television broadcasting in December 2000, demand for digital television BS/CS tuners has been increasing year after year.
However, an existing BS/CS tuner system is constituted by plural parts such as chips and filters, leading to high cost. In order to realize cost reduction, it is important to reduce the number of parts by fabricating the system on one chip, and currently, development of such one-chip tuner system is actively carried out.
In the specification of the digital television BS/CS tuner, the tuner must receive about 20 channels of signals within a broad band ranging from 1 GHz to 2.6 HGz, and detect and demodulate only a desired channel without distortion. In the development of the one-chip tuner system, not only a high-performance external filter that suppresses degradation in distortion characteristics due to an interference wave such a signal from another channel but also an on-chip filter that is lower in performance than the external filter are used, and therefore, lower distortion characteristics are required of the reception unit.
Usually, in a radio communication receiver, two methods as follows are carried out to suppress degradation in distortion characteristics.
One is adding a variable gain function to a mixer or amplifier in a reception unit. In this case, an appropriate output level can be maintained regardless of an input signal level, whereby degradation in distortion characteristics of a latter block at large signal input can be suppressed.
There are various methods for realizing such variable gain function, and one of them includes connecting a variable resistor between source terminals of a pair of differential input transistors in a differential amplifier circuit, and controlling an amount of feedback of the amplifier circuit according to the resistance. Further, there is another method described in, for example, Japanese Published Patent Application No. Hei. 8-256039 (pages 1-7, FIG. 1). In this method, a variable resistor is constituted by two variable resistors connected in series, each using a channel resistance between a source and a drain of a MOS transistor, and an appropriate bias voltage is applied to a node of the two MOS transistors to feed a DC current through the node, thereby to operate the variable resistor so as to avoid a point at which the resistance of the channel resistor steeply changes, and thus degradation in distortion characteristics during gain control is suppressed.
The other method is inserting a filter in a reception part as mentioned above. The filter attenuates unnecessary signals such as an interference wave signal and an intermodulation distortion, whereby degradation in distortion characteristics in the later block can be suppressed (for example, refer to “RF CMOS Circuit Design Technique” written by Nobuyuki Ito, Triceps Co. Ltd., June 2002, Pages 12-23).
Furthermore, an output load part of a mixer or an amplifier may be provided with a filter function. To be specific, a capacitor is connected to an output terminal of a mixer or an amplifier to constitute an LPF (Low Pass Filter) by the output load impedance and the capacitor. Thereby, unnecessary signals such as an interference wave signal and an intermodulation distortion are attenuated, whereby degradation in distortion characteristics of the mixer or amplifier itself or distortion characteristics of the later block can be suppressed.
Hereinafter, a description will be given of a conventional variable gain amplification circuit in which a capacitor for realizing a filter function is connected to an output terminal of a mixer having a variable gain function.
FIG. 14 is a diagram illustrating the conventional variable gain amplification circuit, wherein 101 denotes a variable gain mixer using a variable output load impedance, 102 denotes an RF signal source, 103 denotes an LO signal source, 104 denotes an output terminal, 106 denotes a control circuit, and 112 denotes a capacitor.
Hereinafter, a description will be given of the operation of the conventional variable gain amplification circuit constituted as described above. The variable gain mixer 101 multiplies an RF signal supplied from the RF signal source 102 and an LO signal supplied from the LO signal source 103 to frequency-convert the RF signal, and outputs an IF signal to the output terminal 104.
The control circuit 106 controls the gain of the variable gain mixer 101 so as to constantly output an appropriate IF signal level regardless of variations in the RF signal level supplied from the RF signal source 102.
The capacitor 112 has a function of attenuating unnecessary signal components such as an interference wave signal or intermodulation distortion that is included in the IF signal, an RF leakage signal, an LO leakage signal, and the like, and the capacitor 112 is connected to suppress degradation in distortion characteristics.
Hereinafter, the function of suppressing degradation in distortion characteristics, possessed by the capacitor 112, will be described in detail.
FIG. 15 is a diagram illustrating, as an AC equivalent circuit, the variable gain mixer 101 of the conventional variable gain amplification circuit shown in FIG. 14, wherein 113 denotes an IF signal source, and 114 denotes a variable output load impedance.
In FIG. 15, it is apparent that a filter is formed when paying attention to the variable output load impedance 114 and the capacitor 112.
Assuming that the impedance of the variable output load impedance 114 is Z14, and the capacitance of the capacitor 112 is C12, a transfer function A of the filter viewed from the IF signal source 113 is expressed by formula (1) as follows.
                    A        =                  1                      1            +                                          j                ·                ω                ·                C                            ⁢                                                          ⁢                              12                ·                Z                            ⁢                                                          ⁢              14                                                          (        1        )            
First of all, a description will be given of the case where the variable output load impedance 114 is a variable resistor, that is, the variable gain mixer 101 uses a variable resistor in the output load part.
Assuming that the resistance of the variable output load impedance 114 is R14, a transfer function is expressed by formula (2) using formula (1).
                              A          ⁢                                          ⁢          R                =                  1                      1            +                                          j                ·                ω                ·                C                            ⁢                                                          ⁢                              12                ·                R                            ⁢                                                          ⁢              14                                                          (        2        )            wherein AR is a transfer function of a primary LPF, having frequency characteristics shown in FIG. 16. In FIG. 16, fc is a cutoff frequency that is given by formula (3).
                              f          ⁢                                          ⁢          c                =                  1                      2            ⁢                                                  ⁢                          π              ·                                                          ⁢              C                        ⁢                                                  ⁢                          12              ·              R                        ⁢                                                  ⁢            14                                              (        3        )            
Accordingly, it is possible to attenuate unnecessary signals such as an interference wave signal, an intermodulation distortion, an RF leakage signal, an LO leakage signal by setting the capacitance C12 so that the cutoff frequency fc becomes a maximum frequency that does not attenuate a desired IF signal. As a result, degradation in distortion characteristics can be suppressed.
Next, a description will be given of the case where the variable output load impedance 114 is a variable inductor, that is, the variable gain mixer 101 uses a variable inductor in the output load part.
Assuming that the inductance of the variable output load impedance 114 is L14, a transfer function AL is expressed by formula (4) using formula (1).
                              A          ⁢                                          ⁢          L                =                  1                      1            -                                                            ω                  2                                ·                C                            ⁢                                                          ⁢                              12                ·                L                            ⁢                                                          ⁢              14                                                          (        4        )            wherein AL is a transfer function of a secondary LPF, having frequency characteristics shown in FIG. 17. In FIG. 17, fr is a resonance frequency that is given by formula (5).
                              f          ⁢                                          ⁢          r                =                  1                      2            ⁢                                                  ⁢            π            ⁢                                          C                ⁢                                                                  ⁢                                  12                  ·                  L                                ⁢                                                                  ⁢                14                                                                        (        5        )            
Accordingly, it is possible to attenuate unnecessary signals such as an interference wave signal, an intermodulation distortion, an RF leakage signal, and an LO leakage signal by setting the capacitance C12 so that the cutoff frequency fc becomes a maximum frequency that does not attenuate a desired IF signal. As a result, degradation in distortion characteristics can be suppressed.
However, in the construction of the conventional variable gain amplification circuit shown in FIG. 15, since the variable output load impedance 114 of the variable gain mixer 101 varies during gain control, the cutoff frequency fc or the resonance frequency fr varies, whereby unnecessary signals such as an interference wave signal, an intermodulation distortion, an RF leakage signal, and an LO leakage signal cannot be sufficiently attenuated, resulting in degradation in distortion characteristics of the variable gain mixer 101 or the latter block.
FIG. 18 is a diagram which plots tertiary distortion intercept points (IIP3) against gain setting in the conventional variable gain amplification circuit shown in FIG. 15, representing that the distortion characteristics are degraded at the lower gain side.
When the gain is set low, the control circuit 106 performs control so that the variable output load impedance 114 is lowered. Therefore, the resistance R14 in formula 3 or the inductance L14 in formula 5 is reduced, and the cutoff frequency fc or the resonance frequency fr is increased. As a result, a tertiary intermodulation distortion (IM3) signal that is not required cannot be sufficiently attenuated, resulting in degradation in distortion characteristics at the lower gain side as shown in FIG. 18.